[Rust] match demo

fn main() {
    let grade = "A";

    match grade {
        "A" => println!("Wonderful"),
        "B" => println!("Good"),
        "C" => println!("So so"),
        "D" => println!("It could be better"),
        "F" => println!("Oh, oh"),
        _ => println!("Unknown grade"),  // Error on removal.
    }
}